This invention relates to CMOS output buffers, and more particularly to low-voltage differential-signaling output buffers.
A variety of electronic systems rely on improvements in high-speed signaling to increase performance. Networks can operate at higher data rates when signals are propagated more quickly between stations and within network switches. Computers process data at a higher rate when the data can be sent to memory and peripherals at higher speeds. Telephone systems can handle more calls when faster transmission occurs.
One of the bottlenecks to improved performance is external signaling, when signals are driven from an integrated circuit (IC) to another IC. Metal traces on a printed-circuit board (PCB) must be driven by the IC's output buffer. These metal traces are much larger than the metal lines on the IC itself and thus have a higher capacitive and resistive load. Some signals need to be drive off the PCB across cabling, perhaps to other rooms or buildings. An even higher drive is then needed.
Traditionally the IC's output buffers drove signals fully to the power rails--Vcc (or Vdd) and ground. CMOS-level or Transistor-Transistor-Level (TTL) signals were driven from ground to 5 volts or 3 volts and back when transitions occurred. While such TTL or full-level signals are useful to provide wide noise margins, large external loads cannot quickly be driven with such wide voltage swings without creating electro-magnetic interference (EMI) and distortions. Driving low impedance transmission lines has long been a problem.
While full-swing signaling is still used internally in IC's, more recently off-chip signaling has employed limited-voltage swings. Rather than swing the output a full 5 volts, the outputs swing only 350 milli-volts (mV) or so. Two output signals per bit of information are used, rather than just one output. The two outputs are driven to opposite states, in what is known as differential signaling. When one output swings up 350 mV, the other swings down 350 mV. The two differential signals can be connected together by a resistor at the far end of the cable to produce a voltage difference when current-switching output drivers are used.
Such low-voltage differential signaling (LVDS) techniques can speed data rates and reduce EMI, even when long cables are driven.